In our GB published Application (Ser. No. 2,207,525) corresponding to U.S. application Ser. No. 214,603, filed 1st Jul. 1988 (Bricheno et al.) (now abandoned) there is described an N.times.N star coupler fabricated using an annular coherent mixer guide. Whereas this star coupler generally has N inputs and N outputs, it may be used simply as 1.times.N star coupler. An 8.times.8 star coupler according to the aforementioned application may be manufactured by taking eight 125 .mu.m OD single mode fibres which have been progressively stretched using the adiabatic technique described in GB 2,150,703 B (U.S. Pat. No. 4,591,372) to produce parallel-sided reduced diameter regions of 50 .mu.m OD, this increases the mode spot diameter from 10 .mu.m to about 25 .mu.m. These reduced diameter regions of the fibres are clamped around a silica former whose diameter is such that the fibres form a closed packed layer thereon. The resulting sub-assembly is then glued together, sawn in half and the sawn edges polished. The polished end of one half is butted up against one end, the input end, of an annular mixer guide of appropriate dimensions and the other half butted up against its other end in the appropriate orientation. Index matching adhesive is employed to joint the three parts and the resulting assembly may be potted. Injection of a Gaussian spot of light into the annular mixer guide by one of the fibres results in spreading out of the light as it travels down the guide. Initially the power will spread all round the annulus but after a certain distance it begins to overlap back on itself. Since the initial and overlapping light originates from the same source it is coherent and an interference pattern is created. After a certain distance into the annular mixer guide a high contrast regular pattern of output spots appears every few millimeters, the number of output spots gradually decreasing. For the 8.times.8 coupler referred to above and 1.3 .mu.m wavelength light, an annular mixer guide with an OD of 180 .mu.m, a wall thickness of 50 .mu.m and length of 23 mm is required. With a length of 23 mm eight output spots are obtained. The output fibres are aligned with these output spots. An annular coherent mixer guide was used since it gave all input and output fibres an equivalent position and, by fabricating the annulus sufficiently narrow, it will confine the beam in a single mode in one of the input planes. Since the operation of the coupler is interference based, the coupler is inherently wavelength sensitive.
The present invention aims to produce a 1.times.N coupler structure which is such that, in particular, its fabrication can be facilitated in comparison with that required for the N.times.N couplers with annular mixer guides referred to above.